Process for producing white wood pulp of high solution viscosity and purity



Patented July is, 1941 PROCESS FOR PRODUCING WHITE WOOD PULP OF IIIGII SOLUTION VISCOSITY AND runrrr George A. Richter, Berlin, N. 11., assignor .to Brown Company, Berlin, N. E, a corporation of Maine No Drawing. Application February 10, 1936, Serial No. 63,273

2 Claims.

Thisinvention relates to wood pulp of high solution viscosity and more particularly to wood pulp having both high solution viscosity and an alpha cellulose content upwards of 96%. The wood pulp product of the pgsent invention also has the attribute of whitene and is hence especially adapted for conversion into all sorts of cellulose derivatives having high solution'vicosity and requiring a cellulose base of high solution viscosity, high purity, and substantial freedom from color and color-generating substances. For instance, it is excellent raw material for the preparation of cellulose nitrate, cellulose acetate, and other cellulose derivatives to be fabricated into end-products after being dissolved to form high viscosity solutions whose high viscosity quality is attributable to the high solution viscosity of the cellulose nitrate, cellulose acetate, or other cellulose derivative dissolved therein. In this connection, it might be remarked that end-products, such as artificial silk, films, and plastics have been found to possess better physical properties, including higher strength, when fabricated from high viscosity cellulose-derivative solutions, that is, solutions containing cellu lose nitrate, cellulose acetate, or other cellulose derivatives of high solution viscosity dissolved therein, than when fabricated from low viscosity cellulose-derivative solutions.

In order to develop a finished product of high purity and whiteness from ordinary unbleached wood pulps, it has been found necessary to subject such pulps to the action of bleach liquor and of alkaline refining liquor. I have observed that the more powerful the bleaching or refining action of a liquor, the more drastic is its solutioncaustic soda solution of mercerizing strength or activity at room temperature such as is effective in extracting non-alpha cellulose components from the pulp to the desired degree, tends to reduce markedly the solution viscosity of the pulp, particularly when the pulp is initially bleached to the desired whiteness. I have found, however,

viscosity-lowering effect on wood pulp. Accordinstance, when wood pulp is bleached to whiteness as ordinarily and then refined in an alkaline liquor of sumcient alkalinity to increase its alpha cellulose content to a valueupwards of 96%, it is found that the solution viscosity of the pulp has been excessively reduced for the purposes herein desired.

It might be reasoned a priori that it would be impossible to develop a wood pulp product possessed of high solution viscosity as well as whiteness and an alpha cellulose content upwards of that it is possible to produce the desired product and more especially one whose solution viscosity is upwards of 6 C. G. S. units, as determined by a test hereinafter given. While the high solution viscosity pulp of the present invention might be had from various processes, nevertheless, I have found that it may be produced by starting with ordinary chemical wood pulp, such as sulphite, partially bleaching such pulp, partially refining the pulp in a weakly alkaline liquor, bleaching the pulp to the desired whiteness, and finally refining the pulp in a strongly alkaline liquor to the desired alpha cellulose content, provided that all of these treatments are carefully controlled especially as regards the temperatures at which they are conducted, that the composition of the bleach liquors is appropriately chosen, and that oxidizing eflects by air andother agencies in the weakly and strongly alkaline refining liquors are minimized.

I shall now describe in detail a process that may be adopted in realizing the product or the present invention. Unbleached chemical wood pulp, e. g. raw sulphite pulp, may be employed as raw or starting material. The sulphite pulp may be prepared as ordinarily in calcium bisulphite cooking liquor, but, from the standpoint of starting with an easier-bleaching pulp, it may be desirable that the bisulphite cooking-liquor be a sodium base or a mixed sodium and calcium base liquor of higher than usual combined SO: content. Thus, in lieu of the usual calcium bi- "sulphite liquor containing about 1% combined S0: and 5% free $02, a sodium or mixed sodium and calcium base bisulphite liquor may be used with a combined S02 content of 1.2% or greater and a free 50: content of 5 to 7%. The pulp may be produced by cooking chipped wood, such as chipped spruce, under the usual time and temperature conditions.

The raw sulphite pulp is washed and carefully screened fro shives or other woody residue, cement or 0th r mineral particle's, etc.; and it is then partially bleached at a consistency of about 5%, as with about 3% to 5% chlorine, based on the weight of dry fiber. Since high temperature chlorination promotes reduction of the solution 96%, since strongly alkaline liquor, for example, viscosity of the fiber, it is preferable to control the chlorinating temperature so that it does not exceed 30 C. After the chlorine has been substantially consumed by reaction with lignin and other impurities associated with the pulp, the pulp is washed.

The washed, partially bleached pulp is then digested in a weakly alkaline liquor at elevated temperature, for instance, at a consistency of about 5%, with about 6% to 12% caustic soda, based on the dry weight of fiber. The digestion of the pulp may be carried on for a period of about 2 to 6 hours at a temperature up to, but preferably not exceeding, about 100 0., since there is a tendency to lower the solution viscosity of the pulp markedly at higher temperatures. It is well to avoid oxidizing influences on the fiber during its digestion in the caustic soda solution,

as these also tend to lower the solution viscosity e of the fiber. The refined pulp is washed free of the alkaline liquor containing dissolved therein non-alpha cellulose impurities extracted from the pulp. The washed pulp may have an alpha cellulose content of about 92 to 94% and a solution viscosity of about 30 to 60, the particular value depending upon the particular conditions of the previous treatments.

The washed, partially refined pulp is then bleached to the desired whiteness, for-which purpose only about 2 to 3% of sodium hypochlorite bleach, based on the dry weight of fiber, need be used. In order to minimize reduction of the solution viscosity of the fiber during the bleaching treatment, it is desirable that such bleach liquor contain suflicient caustic soda or equivalent alkali to maintain a distinctly alkaline condition in the bleach liquor throughout the bleaching operation, say, about 0.5% caustic soda, based on the dry weight of fiber. The pulp may be bleached at a consistency of about 3 to 5%.and at a temperature not exceeding 30 C. After the bleach has been substantially consumed in bringing the pulp to the desired whiteness, the pulp may be washed, at which stage it is found that bleaching has had inappreciable effect on the alpha cellulose content of the pulp but has caused a reduction in its solution viscosity to a value ranging from 12 to 30, the particular value depending upon the particular conditions of the previous treatments. It might be noted that excessive reduction in the solution viscosity of the pulp has been avoided by bleaching the pulp in two stages, namely, preparatory to its refinement in the alkaline liquor and after such refinement. Were the chlorinating or initial bleaching treatment omitted so as to necessitate the use of about 5 to 6% of bleach to effect the desired whitening of the pulp after its refinement, the pulp would sufler a much greater reduction in its solution viscosity even though bleaching were carried out at a temperature below 30 C. and with the maintenance of a distinctly alkaline condition in the bleach liquor throughout the bleaching operation. a

- The white refined fiber of high solution viscos- K ity is then treated with strongly alkaline refining liquor, for instance, with caustic soda solution of 14 to 18% or even greater strength. This treatment may be performed at a stock consistency of about 5% for a suflicient period of time to raise the alpha cellulose content of the fiber above 96%, say, to 98% or even higher, which result may be had at a temperature of about 20 to 30 C. and be accompanied by mercerization of the fiber on account of the mercerizing activity bond, ledger, and writing papers.

of the solution at such temperature. However,

it is possible to carry out the treatment at a temperature as high as about 50 C., in which case mercerization of the fiber may be avoided while preserving its solution viscosity value greater than 6. The entrainment of air or the presence of other oxidizing agents in the strongly alkaline liquor should be avoided in order to maintain the fiber at the desired high viscosity value. Most of the strong refining liquor may be recovered from the refined fiber in substantially undiluted reusable condition, as by draining, centrifuging, or squeezing it from the fiber, whereupon the fiber may be washed; or the fiber washed in'a counter-current washer from which substantially all of the refining liquor is recovered in sumciently concentrated condition to be reused with the addition of but little fresh caustic soda thereto. The washed fiber has the following characteristics:

The whiteness of the pulp thus produced is as high as or superior to that of the highest grade sulphite pulps sold for use in the manufacture of Yet its alpha cellulose content or purity is markedly higher than that of the highest grade sulphite pulps, since such purity comes from the refinement of pulp in a weakly alkaline liquor and then in a caustic soda solution of mercerizing strength or activity at room temperature. It might be noted that bleached sulphite pulps, as they now occur on the market, have a viscosity ranging from about 2 to 5 C. G. S. units, and that they have an alpha cellulose content of about 86 to 88%. If such pulps are refined to higher alpha cellulose content in alkaline liquors, their viscosity is reduced, the extent of reduction depending-upon such factors as the strength of the alkaline liquor and the temperature and time of the refining treatment and being quite pronounced when the refining liquor is of mercerizing strength or activity at room' temperature. In other words, were it attempted to prepare the product of the present invention by subjecting even the highest grades of bleached sulphite pulps to the action of caustic soda solutions of mercerizing strength at room temperature, it would be found that their solution viscosity drops to values far below that of the product of the present invention. It is'evidently by reason of the fact that the whiteness of the product herein is realized by a number of chemical treatments, that is, by preliminary chlorination or partial bleaching, refinement in a weakly alkaline liquor, and bleaching to the desired whiteness, and by controlling such treatments carefully that it is then possible to attain therein the desired alpha cellulose content by the action of a liquor of mercerizing strength at room temperature while preserving its solution viscosity at in absolute C. G. S. units or poises is determined by measuring the viscosity of a solution of 6 grams of fiber in a cuprammonium solution composed of 225 cc. of 28.6% ammonia water containing 9 grams of so-called copper hydrate powder which-is in reality basic copper nitrate corresponding in composition to the formula C11(NO3)2.3C11(OH)2. The C. G. S. unit is employed because it is definite, denoting a viscosity 100 times that of water at 20 C., wherefore, a Q

cuprammonium cellulose solution of standard composition identifying a fiber as having a solution viscosity of is 1000 times as viscousas water at C. The method of, determining or measuring solution viscosity of cellulosefiber used .5 herein is that described by me in mnchgreater i detail in Industrial and Engineering Ghinistrxi: 1 volume 23, page 136, 1931; and inasmfichas the v description of my viscosity-testing method as giv-" en in that publication affords the {particular cri-' 20 terionor test used herein, itlsj' tofbeffu nderstood 'thatmy reference to such description is intended to incorporate such descriptio as! when; Iclaim:

1. A process of producing white,"highlv refined, wood pulp of comparatively'high solution viscosity, which comprises subjecting unbleached chemical wood pulp to a successionoi chemical refining treatments, including chlorination to partially bleached condition, refinement in hot dilute alkaline liquor, bleaching to whiteness, and refinement in strong caustic soda solution, each of. said treatments being succeeded by washing, both said chlorination and bleaching treatments being conducted at temperatures not exceeding about 30 C. and said bleaching treatment being conducted in bleaching'liquor maintained distinctly alkaline throughout the bleaching operation, said first-named refinement being conducted at a temperature not exceeding about 100 C.,

and said second-named refinement being conducted at a temperature of about 20 to C in caustic soda solution of about 14% to 18% or greater strength.

2. A process of producing white, highly refined, wood pulp of, comparatively high solution viscosity, which comprises cooking wood to the p condition of pulp in acidsulphite cooking I v liquor-of a-combined SOicontent greater than; p andsubjectingthe resulting sulphitel wood- V pulp 't'ora succession oi chemical refining treat- 4 including a chlorination to partially bleached condition, refinement in hot dilute alka lineliquor, bleaching jto whiteness, and refinement in strong caustic soda solution, each of said treatments being succeededby washing, 'both said chlorination and bleaching treatments-being conducted at temperatures not exceeding about 30 C. and said bleaching treatment being conducted in bleaching liquor maintained distinctly alkaline throughout the bleaching operation, said first-named refinement being conducted at a temperature not exceeding about- C., and said second-named refinement being conducted at a temperature of about 20 to 50 C. in caustic soda solution of about 14% to 18% or greater strength.

GEbRGE A. RI'CI-I'I'ER. 

